We are
taught that it is an act of ultimate kindness and respect to bury someone and
place a marker at the site. After a person is buried, of course, we can no longer
participate in burying them. However, even if a tombstone has been erected, we
can participate in the mitzvah of making a marker at a grave, by adding to the
stone. Therefore, customarily, we place stones on top of a gravestone whenever
we visit to indicate our participation in the mitzvah of erecting a tombstone,
even if only in a more symbolic way.